The invention relates to a circuit for the protection by a timed relay of apparatus susceptible to an overload, in general, and more particularly to a protection circuit responsive to the volt-per-Hertz ratio exhibited by such apparatus.
It is desirable to maintain a constant volt-per-Hertz ratio on a load which may vary with respect to voltage and frequency. This is the case with a variable speed AC motor drive. The main reason for maintaining a constant ratio resides in the fact that the flux in the air gap is dependent upon such a ratio.
There are cases where such constant ratio cannot be maintained automatically, and protection against excessive magnetic flux requires protection by a relay to shut down the operation of the load, or apparatus, whenever the ratio deviates from a predetermined level. This is the case with an electrical generator driven by a turbine, because the turbine may at times be operated under conditions which are out of range for the generator, for instance at start-up, or under low load conditions. This is also the case with a transformer exposed to strong voltage changes, or frequency variations. In such extreme situations, a relay would shut-down the electrical generator, or open the power supply to the transformer.
Protection of an electrical apparatus by relay is well known. Generally, upon a critical event the relay is given a delay to respond in order to avoid a shut-down if the event is only temporary and the system had time to readjust to normal operation. In general, depending upon the magnitude of the critical parameters to be protected against, the time delay of the relay is made an inverse function of the magnitude of the critical event.
The problem with a protective relay responsive to a volt-per-Hertz ratio is to provide a time function which matches the criticality of the ratio, in other words, the time delay function, if feasible, should at each operative point substantially meet the overload requirement. Accordingly, it is desirable for the user to have a protective relay itself possessing a timer following a predetermined time function between the controlling event representing input and the effective time delay allowed for tripping. In this regard, ideally, the relay should exhibit a time characteristic in response to the expected range of variations of the volt-per-Hertz ratio. Such characteristic is not easily obtained. In contrast, an hyperbolic characteristic is easily achieved with a RC timing circuit. This, however, does not compare favorably with the characteristic required by the user. A simpler solution in the prior art has been to use a relay having a fixed time delay. This solution provides only one point on the required curve. Using two relays of fixed time delay, will provide two points on the desired curve. One relay of shorter time delay is associated with a higher range of volt/Hz ratio variations and a second relay of longer time delay is associated with a lower range of volt/Hz ratios. Such two-step fixed-time delay relay protection also does not account sufficiently for the required response of the relay.